| Using tracer studies to characterize fluidized bed reactors.
Originally, it was suggested that a reasonable model for a fluidized bed was a PFR
and a perfectly-mixed CSTR in series. We know now that such a model is seriously
in error. The purpose of this problem is to see how such an interpretation could
arise. It is desired to calculate the response of a fluidized bed to cutting off
a steady input of one of the components of the fluidizing gas stream (i.e., a negative-step
tracer experiment). Assume that you are testing the fluidized-bed reactor of Massimilla
and Johnstone, described in Example CD12-4. You will use a steady input of air, containing
0.1% helium. The gas flow rate, temperature, and catalyst loading are all exactly
the same as in the example. At time t =0, the helium flow is cut off. Calculate
and plot the concentration of helium in the outlet gas stream as a function of time.
Does the resulting plot compare what would happen if the same test were interpreted as being a PFR-CSTR series system? If the plot you created were interpreted as being a PFR-CSTR series system, could any parameters in the hypothetical series system be related to critical parameters in the fluidized-bed system? If so, how would they be related? |